#include<cstdio>

double dp[1005][1005];
int main()
{
	int n,s,ns;
	scanf("%d%d",&n,&s);
	ns=n*s;
	dp[n][s]=0.0;
	for(int i=n;i>=0;i--)
		for(int j=s;j>=0;j--)
		{
			if(i==n&&j==s)continue;
			dp[i][j]=(ns+(n-i)*j*dp[i+1][j]+i*(s-j)*dp[i][j+1]+(n-i)*(s-j)*dp[i+1][j+1])/(ns-i*j);
		}
		printf("%0.4lf\n",dp[0][0]);
		return 0;
}
